Glass capillaries, mounting crystals

When the Gandolfi camera is used, a single crystal or crystal fragment is mounted on a glass capillary (Refs 11 19). This camera is thus of great value when a very small amount of the phase to be investigated is available... 

The classical method of mounting crystals is to transfer them into a fine glass capillary along with a droplet of the mother liquor. The capillary is then sealed at both ends and mounted onto a goniometer head (see Fig. 4.20, and Chapter 4, Section III.D), a device that allows control of the crystal s orientation in the X-ray beam. The droplet of mother liquor keeps the crystal hydrated. 

Measuring Raman spectra of polymorphs requires little or no sample preparation. Spectra can be measured from single crystals and from powders. Moreover, samples can be contained in glass capillaries or mounted on a goniometer. As mentioned earlier, fiber optics can be used to interface the instrument with the samples. Thus the measurements are straightforward and easy to perform, whereas the analyses produce information on structures and spectral fingerprints for straightforward identifications. 

Of course, the solubilities of AY, BX, and BY have to be considerably larger than that of AX. As is shown in Fig. 1, the double-infusion crystallizer consists of a thermostated (filled with oil) Pyrex vessel containing the saturated solution and eventually some seed crystals mounted on a stirrer. The solutions AY and BX are introduced through glass capillary tubes by means of a micropump t... 

X-ray diffraction data from crystals are either collected at room temperature or under cryogenic conditions at liquid nitrogen temperatures [around 100°K(-170°C)]. For room temperature data collection, crystals are normally mounted in thin-walled glass capillaries, with a small amount of mother liquor about 5 mm from the crystal. The mother liquor in the capillary is critical because protein crystals are 40-80% water—dried protein crystals do not diffract. The nearby mother... 

FIGURE 7.7. Methods of mounting crystals, (a) A crystal mounted in a glass fiber, as used for a small-molecule crystal that does not decompose on exposure to air. (b) Diagram of the mounting of a crystal in a capillary tube, (c) and (d). A crystal of a chemically modified horse hemoglobin enclosed with mother liquor in a thin walled glass capillary. (Courtesy J. J. Stezowski). 

The crystal should be single, of good diffracting quality, and approximately equidimensional. It is mounted on a gl2iss fiber or, if unstable, in a glass capillary. 

Glue the crystal to a fine glass fibre and place the fiber in a goniometer head. If the crystal is unstable it will need to be mounted in a glass capillary. 

A good diffraction pattern from protein crystals generally requires mounting the crystal in contact with its mother liquor inside a thin-walled glass capillary tube along with a drop or two of the mother liquor so that an equilibrium atmosphere is maintained (Fig. 19). The capillary containing the crystal is then mounted on the goniometer head. Alternatively, a flow cell, in which liquid with a controlled composition is allowed to flow over a crystal, can be used to maintain the required crystal environment. 

For small molecules mounted on a glass fibre, the crystal can often be described by indexing the faces of the crystal and estimating their distance from a common point. In such a case a numerical integration can be used. However, this approach is not valid when the crystal is mounted in a capillary surrounded by mother liquor. 

To prepare the aforementioned single crystals, solutions containing 0.5-1.0 M alkali biphenyl in the appropriate solvents were cooled from 30 °C at a rate of 1 °C/h. Because the anions are sensitive to air and moisture, vacuum techniques, as described in the literature (7) were applied. For the X-ray and magnetic measurements, the crystals were mounted in thin glass capillaries with the aid of a glovebox, which contains less than 1 part per million (ppm) of oxygen and water. Because of the low melting points, the manipulations were carried out at about... 

The molecular structure of 10 was established crystallographicalh. Suitable single crystals were grown from MeCN/EtjO at room temperature over a period of 3-4 days, mounted in a sealed glass capillary and the data were collected at room temperature. The crystal is composed of two crystallographically independent but structurally similar spiral chains. 

Use of single crystal is preferred for structure determination because the data so obtained is easy to interpret. The crystal in this case should be of such a size as is completely bathed by the incident radiation. The crystal is generally affixed to a glass capillary which, in turn, is fastened to a brass pin (Figure 9.26). For protein crystals, however, it is necessary to mount... 

Once a suitable crystal has been obtained, the structure of a molecule of moderate size can often be determined in a day or so. The crystal is mounted on a glass fiber (or in a glass capillary containing an inert gas, if the substance reacts with air) and placed on a pin or spindle within the circular assembly of the x-ray diffractometer (Figure 11.50). The crystal and x-ray detector (placed on the opposite side of the crystal from the x-ray tube) rotate under computer control, while the computer records the intensities and angles of thousands of x-ray reflection spots. After computer analysis of the data, the molecular structure is printed out. 

Crystals that are unaffected by air, moisture, or light are usually mounted directly on a quartz or glass fiber, which is inserted into a goniometer head (Figure 14.3). If the crystal is sensitive to moisture or air, a sealed capillary tube may be used. Suitable apparatus can be used when nonambient temperatures and pressures are needed. The diamond anvil has been developed for high pressure. 

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